Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on September 27, 2019, 05:23:41 pm
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Okay, Im back from the last soldano ish thread. Decided to go a different route. so I'm posting these pics and layout hoping I'll learn something from you guys. All 3 channels work but I have 120hz hum that is LOUD as all get out. I'm figuring a grounding issue but I'm no expert by any means. all parts and relays are brand new as well as the tube sockets and wire. so if any of you guys have time to take a look and tell me where I might be missing it I would appreciate it.
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I'm posting these pics and layout
probably what hung your thread
post a schematic and gut shot.
all 3 ch hummm?
does one do it more or less than the others?
does it change with gain/vol/tone knobs
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Not sure why your original post went wacky?
I could see it in the database, but could not load it into a browser
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yes, all 3 channels hum. its affected by the gain pots and master volumes. clean channel has way less hum but its there. I'm using an elevated heaters supply. all preamp grounds are on a single buss, the tone controls are bussed and then grounded near the input as well as the preamp grounds. the power supply first filter cap in grounded near the power transformer. dropping resisters on on the bottom of the turret board and are 10k except the last one near the last filter cap which is 4.7k.
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more pics
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layout minus tone stacks.
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power transformer seems too close to that rightmost tube with the winding window pointing right at the tube. if you've eliminated all other possible sources of hum (hope you have shields for those tubes), try temporarily moving the PT outside of the chassis with clip leads to see if that helps. you could also try inserting a piece of sheet metal between the PT and the tube row to see if that improves hum, be sure sheet metal is grounded.
relay channel switching in high gain amps is nothing but trouble if not laid out very carefully. the soldano vactrol switching wil be much quieter.
--pete
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power transformer seems too close to that rightmost tube with the winding window pointing right at the tube. if you've eliminated all other possible sources of hum (hope you have shields for those tubes), try temporarily moving the PT outside of the chassis with clip leads to see if that helps. you could also try inserting a piece of sheet metal between the PT and the tube row to see if that improves hum, be sure sheet metal is grounded.
relay channel switching in high gain amps is nothing but trouble if not laid out very carefully. the soldano vactrol switching wil be much quieter.
--pete
I did finally find most of the hum. I grounded the power supply to the preamp buss right at the end on the last tube stages. What's left seems like I can kill it by running the first tube on filtered dc maybe. We'll see. I've been thinking about the power transformer fir a while. Not really sure where to relocate it but may try that as well. For now it has a weird high pitch noise. It's not oscillation I don't think. But maybe I'll post a video later today.
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Okay, so here's where I'm at. Any ideas guys? I appreciate your guys help so much. I have learned a lot here. I'm so grateful for the knowledge you guys share. Here is a video of what I'm dealing with.
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Couple things to try...
Run a 18AWG wire from the power supply filter caps negative side directly to the chassis ground near the input jacks.
Disconnect the AC feed to the relay power supply. I know this will kill any switching. Just interested to see if that is contributing to the hum.
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yes.
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Couple things to try...
Run a 18AWG wire from the power supply filter caps negative side directly to the chassis ground near the input jacks.
Disconnect the AC feed to the relay power supply. I know this will kill any switching. Just interested to see if that is contributing to the hum.
Okay tried this, no difference in hum. the hum is affected by the gain controls and master volumes.
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Coming in late - but is there a schematic for this? Or is it on another thread? From your description I believe you mention hum is present on all channels? Are you sure the unused channels are shunted to ground?
Jim
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Coming in late - but is there a schematic for this? Or is it on another thread? From your description I believe you mention hum is present on all channels? Are you sure the unused channels are shunted to ground?
Jim
There is a layout I posted earlier in the thread. I do have a relay diagram. The input and output of all channels get shunted to ground when not in use.
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In the video, you pointed out the green ground wire that is connected to the ground buss of your board and then terminates at the back of the amp with that cluster of other grounds.
Is that wire also connected to the chassis up near the input jack?
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In the video, you pointed out the green ground wire that is connected to the ground buss of your board and then terminates at the back of the amp with that cluster of other grounds.
Is that wire also connected to the chassis up near the input jack?
The power supply ground runs directly to the preamp ground on the side of the board. Then runs up near the input jack. So it's grounded right at the input. The preamp grounds are basically on a bus and grounded right at the input jack on the chassis. The power supply is grounded to that buzz at the very end last tube stage.
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It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.
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It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.
Can you explain please? How so? The only connection to ground is at the input.
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It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.
Can you explain please? How so? The only connection to ground is at the input.
In the video it looks like you have the left side of the board buss grounded at the input and the right hand side of that same ground buss coming off of the right hand side of the board traveling to the back of the chassis and connected with the main cluster of ground connections.
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It looks like you have created a ground loop by having that "shared" buss connected to 2 points on the chassis.
Can you explain please? How so? The only connection to ground is at the input.
In the video it looks like you have the left side of the board buss grounded at the input and the right hand side of that same ground buss coming off of the right hand side of the board traveling to the back of the chassis and connected with the main cluster of ground connections.
There power supply ground isn't grounded before it gets to that point. It may look that way but the end of the preamp ground on the right side just goes to the negative of the first filter cap.
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There power supply ground isn't grounded before it gets to that point. It may look that way but the end of the preamp ground on the right side just goes to the negative of the first filter cap.
There power supply ground isn't grounded before it gets to that point.
Which point?
That entire cluster at the rear of the amp is the most important ground connection in your amp and needs to be the most SOLID connection under the hood. If it is not super-duper grounded at that point it needs to be.
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So, ground the power supply separate from the preamp grounds? I've been Reading merlins book and he says " The rest of the amplifier will then be connected directly to the terminals of the reservoir capacitor. Connections to any other points on this noisy current loop are not allowed, and no part of this network may be connected to the chassis if a quiet ground scheme is to be maintained! Some very sensitive preamplifiers may even shield this part of the power supply from the audio circuit with a metal bulkhead or screening can."
So now I'm confused. Do I ground the power supply separate from the preamp? Or, ground the power supply and the run the preamp /input ground wire to the power supply ground?
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So, ground the power supply separate from the preamp grounds? I've been Reading merlins book and he says " The rest of the amplifier will then be connected directly to the terminals of the reservoir capacitor. Connections to any other points on this noisy current loop are not allowed, and no part of this network may be connected to the chassis if a quiet ground scheme is to be maintained! Some very sensitive preamplifiers may even shield this part of the power supply from the audio circuit with a metal bulkhead or screening can."
So now I'm confused. Do I ground the power supply separate from the preamp? Or, ground the power supply and the run the preamp /input ground wire to the power supply ground?
As it is now, if you have all of those power supply grounds coming across your board and then the only actual ground connection is near the input jack, that is not a good idea, and could very well be the cause of your issue.
So, you will want to create a main power supply ground at the rear of the amp and attach that to the chassis....i.e. main caps, rectifier
Then you can get rid of the green wire that comes off of the right side of the board and just make sure that the connection you have off of the left side, up near the input is solid.
This is still not an ideal grounding scheme for a ultra high gain monster, but may well get the monkey off your back.
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Thanks so much. What would you suggest as a grounding schem for something high gain like this ? If it were you? I'm just trying to understand and learn stuff.
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I like to think in Guitar amps we use 3 grounds
1st ground is power plug grounded where it enters amp
2nd ground is power supply ground. PI, PA tubes, Rectifiers and associated caps. gets a bolt ground at the PT mounting bolt.
3rd ground is preamp ground, preamp tubes and filter caps for DC gets grounded at input jack
the chassis becomes the "buss" in which they all gets back to the beginning
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Thought I'd post this info maybe it will illuminate something I'm missing and BTW guys, I appreciate all your thoughts and expertise.
Ground reference for elevated heater to first filter cap
Ground from relay power supply
to first filter cap
Ground for the relays them selves to the switches then
To first filter cap
28v from center tapped secondary (used For relay power)
to first filter cap. also including a schematic for the circuit I'm using.
Second filter cap
To first filter cap
First filter cap then goes to ground with one wire.
Preamp bus is grounded near input jack on chassis lug
Relay signal grounds are grounded near input jack on chassis lug
Input grounded near input jack on chassis lug.
All tone control/gain pot ground on a buss wire also grounded on the ground lugs near the input jack.
Hums like crazy. Ground loop?
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Post your schematic in this thread.
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I haven't been working from a schematic per say but I'll try to drawn one up this evening and post it.
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More of a question for me... Should the switching supply be floated?
Thanks!
Jim
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More of a question for me... Should the switching supply be floated?
Thanks!
Jim
That's a good question. I could rebuild the relay power supply with a bridge rectifier instead of what I did. Buthe I'm wondering if I could still use the 28v center tapped secondary by not connecting the center tap and doing an artificial center tapped with 2 100ohmy resisters? Then float the relays? However when I removed power to the relays, it defaults to the clean Channel and still hums. So I dunno.
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Haha! Well I guess that answers my question!
Have you tried removing tubes to see when it first appears?
Jim
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Haha! Well I guess that answers my question!
Have you tried removing tubes to see when it first appears?
Jim
I have, its all there all the way till i pull the last tube to varying degrees.
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Have you tried isolating each channel from the switching circuit - just to abso-positively-everafter rule out the switching circuit? I just cant shake that... :think1: In any case you can check it off the troubleshooting list.
Jim
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An accurate schematic would be much more helpful than a layout.
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Fair enough. I will try to find a way to draw one out.
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Did you remove the ground loop and did that help?
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The forum is not letting me post jpeg images. The file isn't so big so, I guess I'll try pdf for the schematic I did. Hopefully it will help.
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Schematic looks good. (as a piece of art. I haven't actually looked it over)
-Show us where the relay power supply comes from (i.e...separate transformer, filament winding, etc.?)
-And then show us how you have the filaments referenced to ground.
-And how you have the relay power supply grounded.
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Good schematic. I have a plan. DIVIDE AND CONQUER!
Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.
At this point the only things between the input and output are V1, V4, and V7. Does it hum?
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Her is an updated schematic with filament and relay ground.
Schematic looks good. (as a piece of art. I haven't actually looked it over)
-Show us where the relay power supply comes from (i.e...separate transformer, filament winding, etc.?)
-And then show us how you have the filaments referenced to ground.
-And how you have the relay power supply grounded.
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Good schematic. I have a plan. DIVIDE AND CONQUER!
Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.
At this point the only things between the input and output are V1, V4, and V7. Does it hum?
I'll give it a try and let you know. I also posted a NEW updated schematic. All of the relay signal grounds get grounded at the preamp grounds and the relay power grounds are grounded at the main power supply first filter cap. all signal grounds are grounded near the input jack on the chassis.
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Good schematic. I have a plan. DIVIDE AND CONQUER!
Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.
At this point the only things between the input and output are V1, V4, and V7. Does it hum?
Tried this,worse hum and buzz on the clean channel.
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Found error on my schematic, it goes v3a and v2b. It should be v3b.
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Here's what Ritchie200 was suggesting
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Here's what Ritchie200 was suggesting
Would I need to change the rectifier to a bridge rectifier? Also, the center tap for the relay secondary on the trans former just goes to the floated ground on the relay power supply? Just want to make sure before I change stuff around.
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Good schematic. I have a plan. DIVIDE AND CONQUER!
Break the circuit at the four points I've indicated.
Add a jumper (doesn't have to be shielded) between pot wiper and .22µF cap.
At this point the only things between the input and output are V1, V4, and V7. Does it hum?
Tried this,worse hum and buzz on the clean channel.
So, you have bad hum even with this simple arrangement. I would leave it connected just like this and solve the hum issue. Once solved for this arrangment, the hum will probably be solved for the whole circuit.
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Would I need to change the rectifier to a bridge rectifier? No
Also, the center tap for the relay secondary on the trans former just goes to the floated ground on the relay power supply? Yes, as it appears in my little schemo snip
BUT, stick with sluckey's advice. Be thankful he is here. You have something mis-wired or improperly grounded and my wish for you is that you will eventually find it.
My original concern was that you had 5-6 ground wires connected to a terminal and then 1 wire to carry all of that over to your preamp board.
The fact that you thought that was a good idea leads me to believe that there might be another bad idea in there somewhere. :icon_biggrin:
Take sluckey's CAPS as gospel
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DIVIDE AND CONQUER AGAIN. Let's simplify even more. Remove the jumper between that pot wiper and that .22µF cap. Now the only thing in the circuit is V7. Does it hum?
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The fact that you thought that was a good idea leads me to believe that there might be another bad idea in there somewhere.
Take sluckey's CAPS as gospel
I thought that since all of the the tone control terminate at the output stage I needed to ground them all to that stages cathode resister.
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When I do a schematic and layout for an amp build, I include indicating where all the ground wires connect to each other and their B+ filter.
Below I attached a schemo and layout drawing of a build I did. It shows a wired ground star ground system.
I'm thinking where you have that group of B+ filter caps bunched and grounded together and then ran a single wire over and where you have your relay's grounded and relay ac signal grounds are goofed up. I think those relay ac signal grounds should go back to the tube stage ground star they belong to. And I'm thinking those relay switch grounds should not be grounded with the tube stage ac grounds. They should be grounded with the relay power supply.
No one can tell where you have all your grounds wired up and what their wired up to. If you had drawn out the ground stars then we could figure it out.
It's a fairly complicated build with 3 channels and all the relay switching. A lot of grounds to interact with each other as loops. Plenty of places for bug a boos to sneak in.
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The filter cap grounds are locally decoupled. In other words they sit on the board and are grounded right at the stage they are supplying power for. There is one ground going from the input all the way to the output in a straight line.the power rail is in red if you look at the layout I posted and the grounds are in green.
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Yes, I saw that, but there's more grounds. There's a -lot- more to that preamp then just that main board.
Each set of controls, volume/tone stack, should not be tied/daisy chained together to 1 wire and then brought over to the main board ground buss.
Each set of controls should be brought over separate to the tube ground on the board it feeds. So there should be 3 separate ground wires, 1 from each set of controls, coming over to the board and going over to their own ground star grouping. This is part of keeping the channels grounding separate, so no ground loop between channels.
And each relay ac signal ground should be brought over separately to the tube it comes from and grounded to that tubes ground star. They shouldn't be grounded just anywhere on the ground buss. They should each stay with the ground star of the tube they are dealing with.
What about those 4 B+ filter caps on the separate board on the back of the chassis? What are those caps for, what do they feed/supply? They need to be grounded to the ground star that they feed B+ to.
The 1 B+ filter cap should be grounded to the negative end of the FWB rectifier and nothing else, let it form it's own little loop. Then you bring a buss wire from that point to the next B+ filter cap ground. Very important.
All these things need to be dealt with separately, and need to be going to their own separate ground star. You have 3 separate channels, each channel needs their grounds to stay separate from the other 2 channels. Then you string the ground stars to a buss that ends at 1 single chassis ground connection. It's gonna be a little tricky to do, but it can be done.
You quoted Merlin on grounding. He clearly shows grouping all the grounds from 1 tube together, a ground star, then a buss wire over to the next ground star.
With your preamp, lets take a tube with controls as an example.
12AX7 grounds;
1. Grid R (= volume control)
2. Tone controls
3. K R (cathode resistor)
4. K bypass cap
5. Relay ac signal turn off (ac signal grounded, which is part of the grid circuit)
6. B+ filter cap node that feeds this tube
Those 6 tie together to form a ground star, nothing else but those 6 wires/component leads at that star. Then buss wire over to the next star.
By doing this you make a ground loop that is small and only loops around itself. And this eliminates other possibly ground loops from being formed.
It's a 'wired ground', not 'random grounds'. It controls the ground loops from modulating each other, other parts of the circuit.
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Okay so first of all, sorry. I have redone the grounds since the first pictures of the chassis. I got rid of the heavy ground wire on the pot grounds. The gain pot grounds for each Channel are grounded to the cathode cap they go to. The 3 different sets of EQ are grounded to a single point which is the cathode of v7. I was unaware that the separate eqs would be grounded to the cathode of the last stage for each channel. I though since they all ended at v7 I should ground them there. So, I need to ground the eq fir say channel 1 to the cathode follower of that stage before it hits the last cathode follower buffer output stage and so on? Also, those caps on the back are the power supply for the preamp. I have the bridge rectifier then two 100uf 350v f&t caps with 220k blender resister tied directly to the bridge rectifier. Then that resavoir cap minus gets grounded to the chassis near the power transformer. The two 100uf caps with bleaders after the choke coil are grounded to the cathode of the last cathode follower. The relay power supply grounds are tied directly to the reservoir capacitor and Bridge rectifier circuit on the chassis at the same point at the reservoir capacitor is grounded. The signal grounds from the relay circuits are grounded at the point in the circuit their signal is coming from. So let's just say where the two High Gain channels are, where they meet to hit the output stage when one of those channels is grounded that ground gets grounded to the cathode of its cathode follower. I hope this makes sense.
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I though since they all ended at v7 I should ground them there. So, I need to ground the eq fir say channel 1 to the cathode follower of that stage before it hits the last cathode follower buffer output stage and so on?
Since V7 has it's own K R and a coupling cap in series just before it, I would try grounding those 3 TS's and 3 volume controls back to the previous tube's K ground.
From V4b/V5b/V6b to V7a you have 3 volume pots, 3 tone stacks and 3 relay's.
And ALL of that is part of V7a's grid circuit.
That's a lot of grid wire, the grid is very sensitive and can act like an antenna and pick up all kinds of stray signals. They get injected into the grid along with the ac signal and get amplified.
You might also need some of the grid wires to be shielded wire.
With all that said, go with what Sluckey is telling you to do. (I would.)
DIVIDE AND CONQUER AGAIN. Let's simplify even more. Remove the jumper between that pot wiper and that .22µF cap. Now the only thing in the circuit is V7. Does it hum?
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Here's the most recent schematic. Edited to see if I got the star grounding right? also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.
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Yes, that's what I meant for moving the TS/VC grounds back to the stage before.
Did it kill any of the buzz/hum?
I *think* Sluckey want's you to try just running V7 to see if the PT is causing the buzz/hum, to close to V7?
DL/Pete, brought it up early in the thread. You can try a shield on that tube. It might lessen the buzz/hum, if it does lessen it, then the proximity of PT to V7 is causing the problem.
I'd still try what Sluckey asked you to try, even if you try popping a tube shield on for quick test. (Do what Sluckey asked with the tube shield off.)
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> i wire it exactly like in the picture.
Not sure how literal to take this. But the round-the-bush path of the relay first filter cap return to rectifier WILL hum/buzz. That wire is full of crap. It must go only from rectifier to cap. THEN take amplifier grounds from first cap.
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> i wire it exactly like in the picture.
Not sure how literal to take this. But the round-the-bush path of the relay first filter cap return to rectifier WILL hum/buzz. That wire is full of crap. It must go only from rectifier to cap. THEN take amplifier grounds from first cap.
So, the center tap of the relay transformer to the first filter cap in the supply for the relays. then from the last filter cap to the ht power supply reservoir cap ground? Then I make my switching connection for the relays to that point as well? so, that the relays make their switching connection there? Does that make sense?
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... also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.
"rectifier" or regulator?
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... also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.
"rectifier" or regulator?
Both actually. torched the two diodes as well as the 5v regulator. Saw smoke as it happened. Anyway, im going to build another relay power supply with a full wave bridge. so, can I just wire it just like hoffmans circuit and just use the center tap to ground? or does it need to be floated keeping the center tap un connected?
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... also, tried rewiring the relay power like silver gun said and it smoked my rectifier. i wire it exactly like in the picture.
"rectifier" or regulator?
Both actually. torched the two diodes as well as the 5v regulator. Saw smoke as it happened. Anyway, im going to build another relay power supply with a full wave bridge. so, can I just wire it just like hoffmans circuit and just use the center tap to ground? or does it need to be floated keeping the center tap un connected?
If you follow PRRs drawing you don't have to switch to FWB.
But, if you are going to use a FWB rectifier you do not want to attach the center tap of that secondary winding to anything. Just tape it off (or attach it to an unused point on your board)and do not use. The DC negative end of your FWB will become your new ground connection for your relay power supply.
Once you do that there is no reason why you can't just float the relay supply.
What is the AC voltage of that secondary winding that is supplying your relay power?
I ask because I thought I remembered you saying that is was 28V, and if so, after rectification you are exceeding the max. V rating of the 7805
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Yeah, 28v. I do however have a 9v non center tapped transformer I can use.
"Once you do that there is no reason why you can't just float the relay supply."
So don't ground anything in the relay power supply even the bridge? Float the bridge ground as well?
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read this link
http://el34world.com/projects/relay_switch.htm
close to the bottom Doug has in BOLD
......do not use chassis for DC minus, just run 2 wires from bridge to relay (paraphrased )
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So, the center tap of the relay transformer to the first filter cap in the supply for the relays. then from the last filter cap to the ht power supply reservoir cap ground? Then I make my switching connection for the relays to that point as well? so, that the relays make their switching connection there? Does that make sense?
Yes.
I should have picked up on this before.
PT CT's should always go directly to the 1st filter caps ground end. Like I described for the B+ power supply.
In the 1st drawing there's 2 paths to ground in the circuit, 2 ground paths = ground loop = buzz/hum. In the 2nd (PRR edited) drawing, there's only 1 path to ground for the circuit.
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So in this picture it shows that nothing is connected to ground, correct? The minus just goes to the minus of the diodes on the top of the relays or the switch that is making or breaking the connection with the relays thus switching the relay on or off? So the transformer has a floated ground correct and the bridge is basically also floated? I know all this seems obvious to you guys who have been doing this a long time. However I'm a bit confused about this I look at the drawing and it appears that there's a minus but no ground. So is the minus the ground and you just make her break the connection with the diode on the relays with a switch to ground? Or does the switch connect to the minus on the power supply and all the grounds are floated including the the Transformer and the rectifier and the voltage regulator and the capacitors? Cuz that's what it looks like from the diagram.
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Your confusion is coming from your definition and usage of the term ground.
Ground can also be referred to as common and in some instances the term can help us clarify a mis-conception.
The term "common" refers to a common connection for a circuit, and is usually therefore also know as "circuit common".
It is most widely used as a way to represent 0 volts in reference to some positive voltage.
In our case the positive voltage is +5VDC
When we look at this relay power DC circuit and see a (+) and a (-) it is assumed that we know what we are looking at and will assign 0V to the (-) and +5V to the (+)
Floating this supply ensures that we don't have any interaction with any other circuit commons inside the amp. Switching the (-) path to the relay coil is a simple way of enabling the relay. This switching has nothing to do with the audio signal or it's common connection.
All we need to do is get a switched +5VDC to our relay coil to enable it. There is no need to share another ciruit common to accomplish this so it makes sense to just keep it separate. (even though it could work the other way). In your case this makes even more sense to try because you are experiencing a hum that is usually associated with a grounding issue.
By keeping your "relay circuit common" separated from your "signal path common" you are eliminating one variable that could be causing your issue.
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There's absolutely nothing wrong with using that 28V PT just as it is with that conventional 2 diode full wave conventional rectifier. You get exactly the same dcv as you will by switching over to a FWB and only using half the PT. Nothing gained at all.
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Thanks for the correction sluckey.
I got rid of the post to avoid adding confusion.
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So, I built a new power supply for my relay circuit using a 9 volt non Center tapped Transformer using the Hoffman relay schematic. Works fine but did not add or subtract from the hum issues that I have had since the beginning. So then, I decided to just try star grounding the whole preamp. What I ended up with is exactly the same thing the same hum. If I remove the input tube and both of the High Gain channels first tube the hum goes away. Seems to be affected by the gain pots of those channels. When I turn the gain down hunter gatherer of those channels and their volumes up there is no hum it's just associated with those three tubes the input tube which if I pull it the hum is still there. However when I pulled the two tubes for the high gain the hum goes away like I said. Star grounding made no difference at all.
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Sorry if I'm offtopic, but allow me to give you some advices/opinions.
I don't know if other colleagues suggested you this, but have you tried to rectify the heaters voltage?
In my Soldano Sp77 clone I supplied the heaters by using a full wave rectifier with a 6800uf filter capacitor. You have 7 tubes, so you better use a chassis-mounted 25A bridge rectifier.
Also, the connection between input jack and first triode of V1 needs to be short as possible and using shielded cable. You can solder the 33k gride resistor directly to the socket pin of the tube.
From what I see in the pictures and video, your input jack wire goes to the turret board and then to the grid with a simple wire; please correct me if I'm wrong.
Signal wires between relays and turret board/pots panel also needs to be shielded.
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Also, the connection between input jack and first triode of V1 needs to be short as possible and using shielded cable. You can solder the 33k gride resistor directly to the socket pin of the tube.
From what I see in the pictures and video, your input jack wire goes to the turret board and then to the grid with a simple wire; please correct me if I'm wrong.
Signal wires between relays and turret board/pots panel also needs to be shielded.
Yes.
Grid wires can pick up all kinds of stray signal noise.
Compounds the problem with hi-gain preamps.
I do see some shielded wire runs in the 1st pic you posted. You might need a few more in places. The tubes you pulled yesterday and the amp stopped buzzing, those grids and relay signal wires if more than a few inches might need to be shielded.
You only connected 1 end of the shield to ground right, not both shield ends?
I'd do that before rectifying the heaters. That can have it's own problems introduced.
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Sorry if I'm offtopic, but allow me to give you some advices/opinions.
I don't know if other colleagues suggested you this, but have you tried to rectify the heaters voltage?
In my Soldano Sp77 clone I supplied the heaters by using a full wave rectifier with a 6800uf filter capacitor. You have 7 tubes, so you better use a chassis-mounted 25A bridge rectifier.
Also, the connection between input jack and first triode of V1 needs to be short as possible and using shielded cable. You can solder the 33k gride resistor directly to the socket pin of the tube.
From what I see in the pictures and video, your input jack wire goes to the turret board and then to the grid with a simple wire; please correct me if I'm wrong.
Signal wires between relays and turret board/pots panel also needs to be shielded.
I do have shielded wire going to the input of the first tube. I have a 33 K resistor going from terminal strip directly to the input of the first tube. The two relays that are going to The High Gain channels are strictly grounding relays. All they do when switched on is ground the inputs of those tubes so the clean Channel doesn't have crosstalk. They carry no signal really except to ground the signal out. The three relay board does carry signal but it's just a ground out the output of the two high gain channels. I could use shielded wire on those those runs. And maybe I need to use shielded wire on the two tubes that seem to be causing the noise problems to the grids cuz I currently I'm not.
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The two relays that are going to The High Gain channels are strictly grounding relays. All they do when switched on is ground the inputs of those tubes so the clean Channel doesn't have crosstalk. They carry no signal really except to ground the signal out.
The inputs are the grid. Your grounding the grid. That grid grounding wire is an antenna when it's not grounded, doesn't have to carry signal from 1 point to another. The longer it is, the larger the antenna, picks up more. Should be shielded.
The three relay board does carry signal but it's just a ground out the output of the two high gain channels. I could use shielded wire on those those runs.
After the tone stacks volume controls, those 3 wire runs go to the grid of V7, that is all antenna. Should be shielded.
And maybe I need to use shielded wire on the two tubes that seem to be causing the noise problems to the grids cuz I currently I'm not.
That's a pretty good clue there.
Start with those grid wires 1st. Install the shielded wire 1 run at a time and test for noise reduction. That way you will hear/know where the problem was and how much it was reduced. It might take several shielded wire runs to get it all.
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If the wires with the red arrows are grid/relay ground wires, they need to be shielded. Those are long wire runs, hi-gain makes it worse. Two of those white wires on the board go to blue wires that go to the tube, very long. The 3rd white wire goes to a brown/orange wire to a tube, also pretty long run.
Don't use those turrets/eyelets on the board for the shielded wire runs. Run the shielded wire in 1 piece keeping them as short as possible with in reason. You might be able to use the turret as a tie point for a zip tie?
Most, if not all, of any grid/relay ground wires in the yellow circle are long enough to need shielded wire.
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What is the white wire under the board, red arrow.
What are the 3 zip ties holding under the board, yellow circle?
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"What is the white wire under the board, red arrow"
If you look at the original layout, it show that those wires run under the board to the tone controls. I had asked at one point if having the tone controls so close to the gain controls might be the culprit. However. The white wires from that end if the board carry the signal back to the tone controls. I did try shielding the 2nd tubes grid runs and it made no difference.
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Okay, I think I've hit rock bottom on this thing. I've tried everything I can think of. Shielded grid wires, shielded all long runs, changed tubes, star grounded the preamp, buss grounded it, with relays, without relays, moved the transformer, absolutely nothing changes the hum. It is just there no matter what I do. I'm at a complete loss for what to even try next. The Transformer itself hums but so do others I have in other amps I own but they don't make the amp hum. I'm all out of ideas. I can't even get one single channel to work without hum. You guys got any thing else I could try? Thanks.
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The Transformer itself hums
my last build had a hummer PS, nothing "fixed" it, swapped it out, walla
that hummer is now breadboard ONLY. :dontknow:
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Disconnect the power cord green wire from chassis. Still hum?
What is the output plugged into?
Plug a headphone into the output jack. Still hum?
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Disconnect the power cord green wire from chassis. Still hum?
What is the output plugged into?
Plug a headphone into the output jack. Still hum?
Tried unhook ing earth ground, no difference. Tried head phones, no difference. It's plugged into an amp front but, I have an ebtech hum eleminator as well. Tried it both ways, same hum. So, I don't think it's hum between the two pieces of gear.
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Try connecting the output to a power amp. Any better?
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Okay, just made an interesting discovery. Coming off the input stages, I decided to try an experiment. So coming off the coupling cap where it has a 2meg to ground, it connects to all three gain pot circuits. I lifted the coupling caps on all three, one at a time. Here's the weird. I'm still getting guitar signal on all three channels. No distortion but clean on all three. Way less on the clean channel but pretty loud on both distortion channels. This should have killed all the signal entirely because nothing is connected to push signal down the line.
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This area? What coupling caps?
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This area? What coupling caps?
Yes, that's it. If I lift the 1000pf on both channels as well as the 470k on the clean, there should be no signal at all and yet, I have signal. If I pull v-1 tube all signal goes away. I'm scratching my head. Also, the gain pots still work as well. More like boosting the gain but not distorting it.
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Has to be wired wrong.
This area here, red square?
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Has to be wired wrong.
This area here, red square?
I hear ya, most definitely something is up. However, I have continuity tested the connections from the .022 off the first plate and after breaking all the connections to the gain pots for all three channels. After the .022uf it should have broken the connection. Yet, I still have signal and no continuity between the .022uf cap and any of the gain pots.
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I think this is his most updated schematic?
http://el34world.com/Forum/index.php?action=dlattach;topic=25122.0;attach=79549
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I don't know how but you may have a 2nd path through the relays?
One or more relay signal path may be wired wrong?
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I would try breaking/disconnecting ALL the relay signal wires, input/output. Then jumper the outputs with a gator clip jumper wire, 1 at a time and see if that kills the signal.
If it does, it's the relays miswired, giving secondary signal path. If not, then the preamp R/C filters are miswired.
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Have to go out shopping for food with my wife.
I think you might get it fixed now. :icon_biggrin:
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Have to go out shopping for food with my wife.
I think you might get it fixed now. :icon_biggrin:
Not so much. Lol. I've removed every connection from the board going to tone controls volume controls gain pots relay switching everything nothing Remains. The only thing that remains is the input jack connected to the V1 tube and the output and yet I still have Guitar signal passing.
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Not so much
what do you have for test equipment?
scope
listening amp
signal source
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Head phones, guitar amps. No scope unfortunately.
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Did you disconnect the 3 wires going from R-1/R-2/R-3 to the .22uF cap feeding V7?
If not, just disconnect either end of that .22uF cap going into V7.
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I did. I decided to try an experiment and just hook up one channel, no relays just straight connection. Same hum. I think I'm going to buy another transformer and try that. I'll keep you guys posted.
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Was there still signal at the output with V7 .22uF cap lifted?
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I think I'm going to buy
just straight connection. Same hum
1ST take to neighbors house AND different Headset, way cheaper than shipping for a 15min test
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You can create an audio probe as in the attached picture and connect it to the different inputs of the stages of the preamp.
Maybe you detect if there are any "noisy" stages in your build.
Regarding the tubes, they're new? you're sure abot their good functionality?
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You can create an audio probe as in the attached picture and connect it to the different inputs of the stages of the preamp.
Maybe you detect if there are any "noisy" stages in your build.
Regarding the tubes, they're new? you're sure abot their good functionality?
I'm going to give this a try and see what I come up with. On a different note, i rewired one single channel of the High Gain. Still has unbelievable amounts of hum. As well as parasitic oscillation now. Shielding from the input and output of a gain pot makes no difference whatsoever to the hum or to the oscillation. It's a complete mystery to me. I've built other preamps that work just fine. It's like this preamp is possessed. LOL
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You can create an audio probe as in the attached picture and connect it to the different inputs of the stages of the preamp.
Maybe you detect if there are any "noisy" stages in your build.
Regarding the tubes, they're new? you're sure abot their good functionality?
How do you use the probe? It may seem obvious to some but it's eluding me. Thanks.
You connect the guitar in the jack, and the capacitor terminal "To circuit (test)" you connect it to the input grid or to the grid stopper resistors from different stages as in the attached picture.
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you can use it "backwards" also, stick a cd player's output (volume controlled handy) into the amps input jack, plug headphones or small amp into "test box jack" and probe the same points to evaluate input signal.
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Okay, made the probe tried it. Volumes from the test tone I used seem to be pretty consistent across all of the points that you showed me.
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Okay, just made an interesting discovery. Coming off the input stages, I decided to try an experiment. So coming off the coupling cap where it has a 2meg to ground, it connects to all three gain pot circuits. I lifted the coupling caps on all three, one at a time. Here's the weird. I'm still getting guitar signal on all three channels. No distortion but clean on all three. Way less on the clean channel but pretty loud on both distortion channels. This should have killed all the signal entirely because nothing is connected to push signal down the line.
This has to fixed.
This could cause the buzz/hum. Even if it's not causing the buzz/hum, the 3 channels signal have to be isolated from each other.
I think it's probably cross wired in the relays.
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Have you tried putting the tube shields on the tubes?
If so any help at all?
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Have you tried putting the tube shields on the tubes?
If so any help at all?
Yeah, no difference.
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Okay, here's a discovery. So as previously stated the Transformer hums. With no signal at all, with the gain pot and volume control all the way down and just the output of the preamp to the front of this amp, there is hum Coming through the amp. I tried lifting the ground, the hum remains. I tried a completely new power supply , the hum still remains. If I unplug the guitar cable from the preamp to the front of the amp the hum goes away. Yet, when I lift the ground from the preamp to the amp the hum remains.
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If I unplug the guitar cable from the preamp to the front of the amp the hum goes away. Yet, when I lift the ground from the preamp to the amp the hum remains.
I consider an amp "hum free" when it's all by itself, nothing jacked in and gain/vol/ts ~~ 80% of max and NO hum.
by "lifting" the ground with nothing plugged in your pre is floating, susceptible to all manner of room/power hums n buzzes
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I hear ya. I ordered a new transformer today. I figured I'll give it a try. I still can't understand how I have hum even when the preamp isn't even turned on. With just a cable plug from the preamp to the front of an amp that's turned on there's hum. Yet, I can plug a guitar into the front of the amp direct and there's no harm whatsoever. I really feel like this preamp is cursed. LOL
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I really feel like this preamp is cursed
when I was "figuring tubes out" I did a lot of quick n dirty builds, about 1 in 8 were evil, n since I trouble-shot for a career the LAST thing I wanted to do to relax was chase sneaky gremlins for FUN, so I'd just reduce to parts, come up with another wheel n move on :icon_biggrin:
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Thanks so much for this Shooter. I try to do my best for layouts but I'm obviously still learning. I've made a few movements twords killing the hum. I'm down to 1 high gain channel like one of you guys suggested. I have very little hum now. Almost enough that I could just move on but I think I want to keep plugging at it. A couple of questions I have though. I have merlins book and in his section on filaments he says that in order to supply 1 12x7 with dc it doubles the current needed for the circuit. So in other words, instead of it taking 300 milliamps of power it takes 600 milliamps of power. Question I had is if you were going to run the entire 12 X 7 tube circuit on DC, what that requires 600ma of power times 7 tubes? So 4200ma? My Transformer is rated for 4000ma of power on the filament winding. So can I get away with running all 7 tubes on DC and it be okay? Or is it just too much to push it that far? Thanks.
I really feel like this preamp is cursed
when I was "figuring tubes out" I did a lot of quick n dirty builds, about 1 in 8 were evil, n since I trouble-shot for a career the LAST thing I wanted to do to relax was chase sneaky gremlins for FUN, so I'd just reduce to parts, come up with another wheel n move on :icon_biggrin:
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running all 7 tubes on DC and it be okay?
if you do this math, I'd probably just go for it :icon_biggrin:
4000/4200*100 = answer in %
Glad you're getting hum free, leaves more time for strumming :laugh:
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Before committing to building this circuit you may want to temporarily connect a 12v battery to the filaments to see if it eliminates hum. Lawn mower, portable generator, kids toy car, even your automobile battery. You may even have a backup battery in your computer UPS or security system.
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I've made a few movements towards killing the hum. I'm down to 1 high gain channel like one of you guys suggested. I have very little hum now.
What exactly did you do to kill the hum?
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I've made a few movements towards killing the hum. I'm down to 1 high gain channel like one of you guys suggested. I have very little hum now.
What exactly did you do to kill the hum?
I moved the input jack ground from the grid ground of the first tube to the chassis ground near the input jack. Im allso running all the tube on filtered dc. I also moved the eq components for the high gain channels farther away from the gain pot inputs and outputs.
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Okay, still working on this preamp. I have lyme disease and had to drop it for awhile cause I've been sick. Anyway. I have very, very little hum. I'm running the heaters on filtered dc. Also elevated the heaters to 78v and am using a humdinger as well to balance the heaters. I'm using a none isolated input jack. The first tube is a parallel triode and that entire circuit node is grounded right at the input jack. The rest of the circuit is fully bussed and grounded from the very end of the buss nearest the right side of the chassis to the very first stage's filter cap which is tied to the input jack ground. Now here's what I can't find. There is a very low leve hum that isn't affected by the volume pots. The minute I plug the preamp output into the amp I'm using even if the preamp isn't powered on creates the hum. I've used an ebtech hum eleminator to kill the hum but it persists. It's just there no matter what I do. Any ideas? Thanks.